There are a number of worn components that can contribute to the sagging rear suspension of your car, but the most likely are the shocks and/or the springs. Usually the shocks go first, and if left uncorrected, you can deform the springs permanently.
The shocks have a gas pressurized chamber to prevent the shock oil from cavitating and becoming filled with gas bubbles, which then changes its viscosity and the shock performance. This feature contributes to the superior handling and ride, which is why Mercedes specifies these shocks as OEM components.
As they get old, the gas can leak out, and like any shock, so can the oil. This reduces the contribution of the shock to the force resisting the weight of the car, in addition to the springs, and therefore it affects the ride height. If the shock is really bad for a long time the spring takes the entire load and can eventually spend enough time in a more compressed configuration, leading to yielding of the coil spring material.
So, you most likely need shocks and if oil is leaking that is visually apparent. Just stick your head in there and look around for wetness. It may help to jack the car up and take off the wheel.
I think there are spacers you can buy to raise the height if the shock replacement does not restore the height fully. The spacers go under the coil spring, and this may be a more affordable solution after the shocks are changed out.
Good Luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)